Filter fabrics made from conventional synthetic fibers, non-limiting examples of which are fibers formed from polyesters, polyamides, polyethylene polypropylene and polyacrylonitrile and glass remove particles from a fluid stream initially by a depth filtration mechanism. Such fibers have relatively regular cross-sections of low surface area. Particulate matter in the fluid stream is trapped in the body of the fabric in the interstices between the fibers thereof, to form a cake in the body of the fabric. It is only subsequent to the formation of the body cake that a surface cake builds up, the surface cake then effectively forming the filter medium. The accumulation of the particles inside the fabric creates an appreciable resistance to the flow of fluid through the filter, and correspondingly results in an increase in pressure drop across the filter.
Fabrics which are composed of membranes laminated onto a fibrous substrate considerably reduce entrapment of the particles within the fabric. However, such membranes have an inherently high flow resistance due to the small pore size thereof, and again their use is accompanied by a high pressure drop across the filter.
There has latterly been made available from Lenzing AG an aromatic polyimide fiber known under the trade name P84. Microscopic examination of these fibers indicates them to have an irregular, heavily profiled structure of significantly greater surface area than that of conventional fibers employed in the manufacture of depth filtration filters. These heavily profiled polyimide fibers also have charged surface sites, which generate strong electrostatic fields.
Filter fabrics constructed from HPEC polyimide fibers are found to intercept particulate matter primarily on the surface thereof in a cake filtration mechanism. Fluid flow through such fabric may be gradually restricted, particularly where the feed includes very fine particles. Such particles penetrate the fabric, and since the path resistance tends to be relatively uniform throughout the depth of the fabric, the probability of entrapment is relatively high.
The P84 polyimide fibers are also very expensive, making them cost-prohibitive except for specialty use in very high temperature filtration operations i.e. 200-250 C.
it is an object of this invention to provide filter cloths having improved efficiency.
It is a further object of this invention to provide filter cloths having a high particulate separation efficiency and accompanied therewith a low pressure build-up.
It is another object of this invention to provide filter cloths that are economic.